KEYS: x509: clearly distinguish between key and signature algorithms

An X.509 certificate has two, potentially different public key
algorithms: the one used by the certificate's key, and the one that was
used to sign the certificate.  Some of the naming made it unclear which
algorithm was meant.  Rename things appropriately:

    - x509_note_pkey_algo() => x509_note_sig_algo()
    - algo_oid => sig_algo

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>